The lithium ion rechargeable battery has been in widespread use as a small and light-weight rechargeable battery having a high capacity and a high voltage, suitable for use in portable equipment such as a cellular phone, a video camera, and so forth. Further, the lithium ion rechargeable battery having a high-output has lately come into widespread use as a power supply for an electric power tool requiring a large amount of power.
As there has been no signs of a slowdown in the trend for portable equipment towards further miniaturization, higher performance, and higher function, as it is expected to minimize and to make it lighter, a higher capacity for the lithium ion rechargeable battery has been sought.
Attempts have been made to improve the performance of each component of the lithium ion rechargeable battery and materials thereof. Among other things, efforts have been focused on improving the density and the capacity of a negative electrode material, which is considered to be a limiting factor in the performance of the lithium ion rechargeable battery, and that spheroidizing of a graphite particle for improving the performance is proposed in Patent No. Jp 2983003.
Further, in Patent Document 5 (JP 7-249411 A), it has been disclosed that for the purpose of obtaining a negative electrode material for a lithium ion secondary battery having a large discharge capacity, pretreatment is applied to a carbonizable material at a pressure not lower than 10 kgf/cm2, and a heat treatment temperature not higher than 600° C. in an inert atmosphere to be subsequently subjected to carbonization treatment at a temperature in a range of about 500 to 3300° C. More specifically, it has been described that carbonization is conducted in a range of 1500 to 3300° C. if the carbonizable material is a material easy to carbonize, and in a range of 500 to 1500° C. if the carbonizable material is a material hard to carbonize.
In Patent Document 6 (JP 11-73963 A), there has been described a negative electrode composite material for a lithium ion secondary battery, wherein a binder is carbonized into a matrix carbon, that is produced by a process comprising the steps of forming a slurry comprised of filler graphite, a carbonizable binder, and a solvent for dissolving the binder into spheres by use of a spray-dry method, obtaining agglomerates by drying the spheres, and subsequently rendering the binder infusible or heating the agglomerates in a range of 900 to 1400° C. in an inert atmosphere without rendering the binder infusible    Patent Document 1 JP 2983003    Patent Document 2 JP 3588354    Patent Document 3 JP 3716830    Patent Document 4 JP 3716818    Patent Document 5 JP7-249411A    Patent Document 6 JP11-73963 A